Probe bonding method having improved control of bonding material

ABSTRACT

In assembly of probe arrays for electrical test, a problem can arise where a bonding agent undesirably wicks between probes. According to embodiments of the invention, this wicking problem is alleviated by disposing an anti-wicking agent on a surface of the probe assembly such that wicking of the bonding agent along the probes toward the probe tips is hindered. The anti-wicking agent can be a solid powder, a liquid, or a gel. Once probe assembly fabrication is complete, the anti-wicking agent is removed. In preferred embodiments, a template plate is employed to hold the probe tips in proper position during fabrication. In this manner, undesirable bending of probes caused by introduction or removal of the anti-wicking agent can be reduced or eliminated.

FIELD OF THE INVENTION

This invention relates to probe assemblies for making temporaryelectrical contact to devices or circuits under test.

BACKGROUND

Probes and probe arrangements for making temporary electrical contact todevices or circuits under test have been in widespread use for manyyears. Accordingly, many aspects of this technology have been developed.Although much of this technological development has focused on detailspertaining to the probes, other aspects of probe technology have alsobeen considered. More specifically, probes are typically attached to aprobe card, or some other form of substrate, and some work has focusedon improvements relating to the probe card/substrate.

For example, in U.S. Pat. No. 6,881,974, a probe card manufacturingapproach which starts by forming blind holes in a substrate and fillingthese holes with an electrically conductive metal is considered. Aftersubsequent processing, part of the metal in the blind holes is exposedto form the probe pins. In U.S. Pat. No. 6,259,261, a probe assembly isconsidered where a selector card can be employed to determine the pinpattern of the probing card. In U.S. Pat. No. 6,566,898, a multi-layerprobe card substrate having an improved thermal expansion match tosilicon is considered. In U.S. Pat. No. 6,586,955, a probe assemblyhaving cavities filled with a low melting point metal, which areindividually electrically connected to probe tips, is considered. Byincluding a molten or near-molten metal section in each probe, metalfatigue in the probes can be alleviated, and cracking can be avoided orrendered less harmful by self-healing.

However, as integrated circuit technology continues to develop, it isnecessary to probe at increasingly fine probe pitch (i.e., reduced probespacing). This evolution can generate problems that have not apparentlyarisen in connection with electrical probing before, and which requirenew solutions.

SUMMARY

One such problem is shown in the example of FIGS. 1 a-d. FIG. 1 a showsa probe assembly 104 having probes 102 fixed in position by a bondingmaterial 106 (e.g., epoxy). This kind of probe bonding approach has beenconsidered in U.S. Pat. No. 7,345,492, issued to the present inventor,and incorporated herein by reference in its entirety. It has been foundthat a wicking problem can arise in connection with this probe bondingapproach, as shown on FIG. 1 b. Throughout this application, “wicking”refers to situations where bonding material ends up being disposedbetween probes at locations (e.g., 108 on FIG. 1 b) outside the mainbonding cavity of the probe assembly. This wicking is highly undesirablebecause it typically interferes with proper probe motion duringoperation.

In investigations to date, wicking has most commonly been observed inirregular probe arrays (e.g., probe array 112 on FIG. 1 d), whereclosely spaced probes that are well-separated from other probes in thearray tend to experience wicking. In general, probes having relativelylarge pitch (e.g., lateral spacing 175 μm or more) tend not to exhibitwicking, while regular probe arrays (e.g., probe array 110 on FIG. 1 c)tend to be relatively well-behaved with respect to wicking (e.g., nowicking seen on a regular probe array having 110 μm probe pitch).However, it is expected that wicking in regular probe arrays is likelyto be a problem for pitches of 100 μm or less. It is presently believedthat wicking of the epoxies presently used for probe bonding occursmainly during curing of the epoxy, because the elevated temperature forcuring causes a temporary reduction in epoxy viscosity. This reducedviscosity enables the epoxy to more freely flow along the probes,thereby exacerbating the wicking problem.

According to embodiments of the invention, this wicking problem isalleviated by disposing an anti-wicking agent on a surface of the probeassembly such that wicking of the bonding agent along the probes towardthe probe tips is hindered. The anti-wicking agent can be a solidpowder, a liquid, or a gel. Once probe assembly fabrication is complete,the anti-wicking agent is removed. In preferred embodiments, a templateplate is employed to hold the probe tips in proper position duringfabrication. In this manner, undesirable bending of probes caused byintroduction or removal of the anti-wicking agent can be reduced oreliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-b show a problem that can arise in connection with probeassemblies having closely spaced probes.

FIGS. 1 c-d show examples of probe arrangements.

FIGS. 2 a-d show steps of a probe assembly fabrication sequenceaccording to an embodiment of the invention.

FIG. 3 shows an approach for prevention of inter-probe wicking accordingto an alternate embodiment of the invention.

FIGS. 4 a-b show steps in a process for evaluating the suitability ofanti-wicking materials for practicing embodiments of the invention.

FIG. 5 shows a probe tip template arrangement suitable for use with apreferred embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 2 a-d show steps of a probe assembly fabrication sequenceaccording to an embodiment of the invention. On FIG. 2 a, a first guideplate 202 is separated from a second guide plate 204 by a spacer 206.Guide plates 202 and 204, in combination with spacer 206, form a guideplate assembly and define a bonding cavity (i.e., the region betweenplates 202 and 204 and surrounded by spacer 206). Guide plates 202 and204 include holes within which probes are disposed.

More specifically, each of probes 208 has a base section and a tipsection. The base and tip sections of one of the probes are shown onFIG. 2 a as 212 and 210 respectively. The base sections of the probesare disposed in the guide plate holes such that the base sections of theprobes pass through the bonding cavity of the guide plate assembly. Byway of example, gaps between the probes and the guide plates at theholes are typically between 5 μm and 40 μm, and lateral probe spacing istypically less than about 150 μm.

Typically, the first and second guide plates have corresponding firstand second hole patterns that are aligned with each other, so thatsubstantially straight probe base sections fit into the guide plateassembly, as shown. In most cases, all of the probe tip sections facethe same way relative to the guide plate assembly, also as shown.

FIG. 2 b shows the result of filling the bonding cavity with a curablebonding agent 214. Such filling can be performed by introducing thebonding agent through guide plate holes such as 205 and 207 on FIG. 2 a.Suitable bonding agents include, but are not limited to: epoxies,thermally set materials, molten plastics and injection moldingmaterials.

FIG. 2 c shows the result of the following steps:

-   a) (optional) providing a probe tip template 220 (supported by a    frame 222) and engaging tip sections of probes 208 with the probe    tip template (e.g., as shown on FIG. 2 c) before adding the    anti-wicking agent;-   b) disposing an anti-wicking agent 218 on a surface of the guide    plate assembly facing the tip sections of the probes; and-   c) curing the bonding agent after disposing the anti-wicking agent.    The cured bonding agent is referenced as 216. Typically, curing is    performed by heating the probe assembly, although other curing    processes can also be employed in practicing the invention. The    bonding agent and curing process are preferably selected such that    the bonding agent viscosity is lower during part of the curing step    than before curing is initiated. This combination of properties    facilitates elimination of bonding agent wicking according to    embodiments of the invention, because wicking is suppressed at    points in the process where it would otherwise be most likely to    occur (i.e., during curing). Suitable materials for the optional    probe tip template include polyimide, ceramics and metals.

FIG. 2 d shows the result of removing anti-wicking agent 218 after thebonding agent is cured (and of removing the probe tip template in caseswhere a probe tip template is employed). Anti-wicking agent 218 can beremoved by vacuuming, and ultrasonic cleaning can be employed as a finalcleaning step.

In the example of FIGS. 2 a-d, anti-wicking agent 218 is a solid powder.It is important that bonding agent 214 not wick into the solid particlesof anti-wicking agent 218. Various methods can be employed, individuallyor in combination, to reduce/prevent such wicking. For example, theparticles of anti-wicking agent 218 can be compacted during deposition.Another method is to select solid powder materials having surfaces thatare not wetted by the bonding agent. For example, talc and starch flourhave been found to be suitable anti-wicking agents for an epoxy bondingagent. Fine powders are typically employed for the anti-wicking agent(e.g., particle size about 3 μm). A powder anti-wicking agent having ˜3μm particles has been found to be effective for blocking epoxy wickingthrough probe to guide plate gaps of about 20 μm. A syringe capable ofdelivering the solid particles (i.e., having a needle bore substantiallylarger than the particle size) has been found suitable for deliveringand compacting the anti-wicking agent. A commercial programmable epoxydispenser can be adapted to control the amount of applied anti-wickingpowder.

FIG. 3 shows a fabrication step relating to an alternate embodiment ofthe invention. FIG. 3 corresponds to FIG. 2 c, except that a gel orliquid anti-wicking agent 302 is used on FIG. 3 instead of the solidpowder anti-wicking agent of FIG. 2 c. In order for a gel or liquid tofunction properly as an anti-wicking agent, it is preferred that theanti-wicking agent and the bonding agent be immiscible. Suitable gel orliquid anti-wicking agents for practicing embodiments of the inventioninclude silicone based gels and lithography compatible masking fluids.After the bonding agent is cured, the anti-wicking agent can be removedby standard lithographic processes, and accordingly it is preferred forgel or liquid anti-wicking agents to be removable in this manner. Insituations where probe assembly fabrication is performed in a clean roomenvironment, it is preferred for the anti-wicking agent to be a gel or aliquid, as opposed to a powder.

FIGS. 4 a-b show steps in a process for evaluating the suitability ofanti-wicking materials for practicing embodiments of the invention. Inthis method, a drop of uncured bonding agent 406 is disposed on a bed ofa candidate material 404 on a substrate 402. The bonding agent is curedand then separated from the bed to provide a bead 408. If the surface ofbead 408 is smooth and has substantially the same shape as the surfaceof uncured bonding agent 406 on FIG. 4 a, then the bonding agent doesnot significantly wick into candidate material 404 as it cures, and socandidate material 404 may be a suitable anti-wicking agent. If thesurface of bead 408 is rough and/or if the shape of bead 408 issubstantially different than the shape of bonding agent 406 on FIG. 4 a,then candidate material 404 (as prepared in this test) is not suitablefor practicing embodiments of the invention. This test can be employedto evaluate the suitability of powder, liquid and gel candidatematerials for practicing embodiments of the invention. It can also beused to evaluate deposition conditions, such as degree of compaction fora powdered candidate material.

FIG. 5 shows a probe tip template arrangement suitable for use with apreferred embodiment of the invention. More specifically, this figureshows a close-up view of a probe tip section 504 passing through a probetip template 502 and having a tip 506. Preferably, tip section 504 isnarrower than tip 506, as shown. The probe can move vertically withrespect to the probe tip template, where tip 506 acts as a limit to keepthe probe tip template from sliding off during processing. Preferably,all of the probes in the probe array have this feature which preventsthe probe tip template from inadvertently sliding off the probes.

One approach for providing such a probe tip template is as follows. Aprobe template can be made from a polyimide sheet (e.g., 25 μm thick)with rectangular or square holes formed by laser drilling. The hole sizeis chosen to be slightly larger than the cross section of probe tipsection 504 and slightly smaller than the cross section of probe tip506, so the probe tip can “click” into the hole with application of aslight insertion force. After assembly and curing of the bonding agent,the template can be removed mechanically by gently pulling it off theprobe tips, or chemically or via plasma etch. When mechanical removal isemployed, the probe tips sometime mechanically “give” and losealignment, so the plasma etch removal method is preferred. Plasmaetching completely dissolves the polyimide template without changingprobe tip alignment.

1. A method for fabricating a multi-probe assembly, the methodcomprising: providing a guide plate assembly including a first guideplate separated from a second guide plate by a spacer, said first andsecond guide plates and said spacer defining a bonding cavity; whereinsaid guide plate assembly includes holes in said guide plates; providingone or more probes, each probe having a tip section connected to a basesection; disposing said base sections of said probes in said holes, suchthat said base sections of said probes pass through said bonding cavity;filling said bonding cavity with a curable bonding agent; disposing ananti-wicking agent on a surface of said guide plate assembly facing saidtip sections of said probes; curing said bonding agent after saiddisposing an anti-wicking agent; and removing said anti-wicking agentafter said curing said bonding agent.
 2. The method of claim 1, furthercomprising: providing a probe tip template; engaging said tip sectionsof said probes with said probe tip template prior to said disposing ananti-wicking agent.
 3. The method of claim 2, wherein said probes engagewith said probe tip template by passing through holes in said probe tiptemplate, and wherein tips of said probes are larger than said holes insaid probe tip template.
 4. The method of claim 1, wherein gaps betweensaid probes and said guide plates at said holes are between 5 μm and 40μm.
 5. The method of claim 1, wherein a lateral spacing of said probesis less than about 150 μm.
 6. The method of claim 1, wherein saidbonding agent comprises a material selected from the group consistingof: epoxies, thermally set materials, molten plastics and injectionmolding materials.
 7. The method of claim 1, wherein said curingcomprises heating.
 8. The method of claim 1, wherein said bonding agenthas a lower viscosity during part of said curing than before said curingis initiated.
 9. The method of claim 1, wherein a first hole pattern ofsaid first guide plate is substantially aligned with a second holepattern of said second guide plate, and wherein said base sections ofsaid probes are substantially straight.
 10. The method of claim 1,wherein all of said tip sections of said probes face the same wayrelative to said guide plate assembly.
 11. The method of claim 1,wherein said anti-wicking agent comprises solid particles.
 12. Themethod of claim 11, wherein said bonding agent substantially does notwick into said solid particles.
 13. The method of claim 11, wherein saidsolid particles are compacted during said disposing an anti-wickingagent.
 14. The method of claim 11, wherein a size of said solidparticles is about 3 μm.
 15. The method of claim 11, wherein said solidparticles comprise talc or starch flour.
 16. The method of claim 11,wherein said disposing is performed with a syringe capable of deliveringsaid solid particles.
 17. The method of claim 1, wherein saidanti-wicking agent comprises a gel or liquid.
 18. The method of claim17, wherein said bonding agent and said gel or liquid are substantiallyimmiscible.
 19. The method of claim 17, wherein said gel or liquidcomprises a lithography-compatible masking fluid or a silicone basedgel.